Hello James Nelson,
Do you by any chance have a large data source for the chart you are displaying? By deault if it is more than 200 items RadChart will sample the data and this may cause individual dataitems to be lost. To disable sampling try RadChart.SamplingSettings.SamplingThreshold = 0.
If that doesn't help please give us more details. You might have found a bug in which case we would really like to get to the bottom of it and fix it. Sending in a project will help a lot. Thanks for your cooperation.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.